The present invention relates to a control system for controlling a motor-driven injection molding machine having an injection unit that is driven by a servo-motor.
Motor-driven injection molding machines have an injection unit that is driven by a servo-motor. The injection unit comprises a ball screw, a nut, and a pressure plate. The ball screw rotates in response to rotation of the servo-motor. The nut is moved forward and backward depending on the rotation of the ball screw either in the normal or the reverse direction. The pressure plate moves forward and backward in cooperation with the nut. A screw provided within an injection cylinder moves forward and backward in response to the forward and backward movement, respectively, of the pressure plate.
As described more in detail below, the servo-motor is controlled by a speed feedback control loop. The speed feedback control loop comprises an amplifier, a motor drive, a rotation speed sensor, and a subtracter. The amplifier receives a speed command value and amplifies it to produce an output torque command value for the servo-motor. The motor drive drives the servo-motor according to the output of the amplifier. The rotation speed sensor senses a rotation speed of the servo-motor as a sensed speed value. The subtracter is provided on the side of an input of the amplifier and subtracts the sensed speed value obtained by the rotation speed sensor from the speed command value to produce a subtraction result to the speed amplifier.
The output of the amplifier that is supplied to the motor drive is a torque command value used to follow the speed command value. In such a case, a torque for acceleration or deceleration is required to accelerate or decelerate the servo-motor. The torque command value is produced by the amplifier, so that it is necessary to provide a difference between the speed command value and the sensed speed value, i.e., a speed deviation, to the amplifier. This means that the speed deviation is present when the servo-motor requires a torque. Thus the speed deviation can be used as an index for response of the speed feedback control loop.
A smaller speed deviation is preferable for better response. In principle, the speed deviation can substantially be zero if the amplifier has an infinite gain. However, it is impossible in practice. To cope with this, the gain of the amplifier may be increased to eliminate a response delay due to the speed deviation. However, a too much larger gain results in instability of the control, which adversely affects the quality of resultant molded products.